1. Field of the Invention
This invention generally relates to a polymeric material useful for coating an implantable device, such as a stent.
2. Description of the Background
Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, stent thrombosis remain. Pharmacological therapy in the form of a drug-delivery stent appears a feasible means to tackle these biologically derived issues. Polymeric coatings placed onto the stent serve to act both as the drug reservoir, and to control the release of the drug. One of the commercially available polymer coated products is a stent manufactured by Boston Scientific. For example, U.S. Pat. Nos. 5,869,127; 6,099,563; 6,179,817; and 6,197,051, assigned to Boston Scientific Corporation, describe various compositions for coating medical devices. These compositions provide to stents described therein an enhanced biocompatibility and may optionally include a bioactive agent. U.S. Pat. No. 6,231,590 to Scimed Life Systems, Inc., describes a coating composition, which includes a bioactive agent, a collagenous material, or a collagenous coating optionally containing or coated with other bioactive agents.
The nature of the coating polymers plays an important role in defining the surface properties of a coating. For example, a very low Tg, amorphous coating material induces unacceptable rheological behavior upon mechanical perturbation such as crimping, balloon expansion, etc. On the other hand, a high Tg, or highly crystalline coating material introduces brittle fracture in the high strain areas of the stent pattern. Furthermore, a very low Tg, amorphous coating material can have a high drug permeability leading to an unacceptably high drug release rate. While a high Tg, or highly crystalline coating material can have a very low polymer permeability, which lead to an unacceptably low drug release rate. These are general principles which must also be combined with the properties of the drug such as the drug solubility in the polymer and in the tissue.
Some of the currently used polymeric materials have some undesirable properties such as lack of sufficient elongation to use on a stent or low permeability to drugs. One such polymer is poly(vinylidene fluoride) (PVDF). Therefore, there is a need for new polymeric materials suitable for use as coating materials on implantable devices.
The present invention addresses such problems by providing a polymeric material for coating implantable devices.